1. Field of the Invention
The present invention relates to a photopolymerization method, an ink set, an ink composition, and a water-soluble biimidazole.
2. Description of the Related Art
As an image-recording method in which an image is formed on a recording medium such as paper on the basis of an image data signal, there are an electrophotographic method, a sublimation-type or melting-type thermal transfer method, an ink jet method, and the like.
In the ink jet method, the operation cost is inexpensive because a plate is not required during printing, and an ink composition can be efficiently used by directly forming an image on a recording medium by discharging the ink composition only in necessary image sections. Furthermore, in the ink jet method, an inexpensive printing device is used, and only a small amount of noise is generated. As described above, the ink jet method has a variety of advantages compared with other image-recording methods.
As an ink composition used in the ink jet method, a radiation curable ink composition is known. The radiation curable ink composition is cured by the polymerization of a polymerizing component in the ink composition through radiation of a radioactive ray such as an ultraviolet ray, and thus has an advantage that ink is not easily bled from an image compared with a case in which a radiation non-curable ink composition (a solvent-based ink composition) is used.
In recent years, there has been a demand for use of an LED as a UV light source for curing the radiation curable ink composition from the viewpoint of energy saving and space saving. However, α-hydroxyacetophenone-based photopolymerization initiator, which is a photopolymerization initiator widely used for radiation curing, has a short (approximately 280 nm) absorption wavelength, and thus is almost incapable of polymerizing an ink composition with a UV-LED light source (approximately 365 nm).
Meanwhile, in a system in which a biimidazole-based photopolymerization initiator is used, it has been reported that the sensitivity is improved by combining the biimidazole-based photopolymerization initiator with an amine-based, thiol-based, or disulfide-based hydrogen donor (refer to JP2000-98124A, JP2003-206307A, and Chem. Abstr., 1967, 66, 50700).